This invention relates generally to rotating electric machines, and more particularly to a method and apparatus for making a permanent magnet rotor for an electric motor.
One type of rotating electric machine, particularly a motor, includes a permanent magnet rotor rotatably mounted in a cylindrical stator. The stator forms a rotating field which turns the rotor. Typically the rotor includes a number of permanent magnet segments fastened to an iron core such as a stack of laminations, and E. J. Schaefer U.S. Pat. No. 4,742,259 shows a rotor of this nature.
A problem with a rotor of this type has to do with the fastening of the magnet segments to the core. The segments must be secured in place because the centrifugal force during high speed rotation tends to throw them off, and the inertia and turning forces on the segments tend to move them circumferentially.
Numerous arrangements have been provided for securing the segments in place. Adhesives have been used but are not satisfactory because of the expansion and contraction of the parts due to temperature variations and, in the case of liquid filled motors, adhesives are not available which will hold up over a long period of time in a wet environment. Various types of wedges and fasteners have been used but they generally result in a complex and expensive structure.
A common method has been to form a cover or sleeve over the outer side of the segments. Covers made of a synthetic material have been provided but they are not believed to be as strong as metal or as resistant to the liquid of a filled motor. A metal sleeve or cylinder over the segments, as shown in the above-mentioned Schaefer U.S. Pat. No. 4,742,259, should be very thin because it is located in the air gap between the rotor and the stator and it should not materially interfere with the field in the air gap. In the instance of a water-filled submersible motor, a cylinder made of thin stainless steel is preferred.
In a rotor including a thin metal sleeve or cylinder over the segments as discussed above, the cylinder should have a residual tension so that it applies a strong holding force on all of the segments. A problem in this connection arises from the nature of the currently available high performance permanent magnets. The magnets are molded of a ceramic material which is very hard, and the molding process results in variations in the radial thicknesses of the segments. One way to make the outer cylinder engage and hold all of the magnet segments would be to grind the segments to an exact thickness, but again this is a relatively expensive process.
In a prior art method, a metal cylinder has been mounted over the segments by initially making the cylinder's internal diameter slightly less than the external diameter of the magnet segments. The cylinder is heated to expand it and it is then placed over the segments and cooled so that it is shrunk over the segments. This method produces the residual tension in the cylinder but it is a relatively difficult process.
The D. W. Jones U.S. Pat. No. 4,759,116, dated July 26, 1988 discloses an arrangement wherein fluid pressure is applied internally of a cylinder in order to expand it, and then a metal core and magnet segments on the core are inserted into the expanded cylinder. When the internal pressure is removed, the cylinder contracts and grips the segments. This arrangement requires a relatively complex sealed chamber arrangement.
The K. Okamoto U.S. Pat. No. 4,777,717, dated Oct. 18, 1988 discloses a method of mounting magnet segments in a cylindrical yoke of a motor. The yoke is first plastically deformed by internal expander elements, the elements are removed, and the magnet segments are inserted in place of the elements. There is no residual tension produced by this method and a two-step process is required.
It is a general object of this invention to provide an improved method and apparatus for assembling a cylinder with magnet segments of a rotor and producing residual tension in the cylinder.